Osteoporosis remains a major cause of morbidity and mortality. Parathyroid hormone (PTH) is the only currently available therapy that substantially increases bone mass, but this therapy's effectiveness wanes with time. Our lack of understanding of the limitations of PTH therapy reflects very incomplete understanding of why PTH increases bone mass. We need to understand better the cellular targets of PTH and the intracellular mediators of PTH action. We propose to clarify both the cellular targets of PTH action and to identify a previously missed link in the action of PTH on osteocytes. In Aim 1, we propose to characterize early cells of the osteoblast lineage in vivo. These cells are likely PTH targets (direct and indirect), but our ability to identify these cells and study how PTH changes them in vivo is limited. We will mark such early cells in the osteoblast lineage with a collagen 11-creERt transgene that will activate expression of a red fluorescent marker (tandem dimer tomato). We will determine the fates of such cells (how they differentiate into osteoblasts and adipocytes, how they proliferate and die, whether they participate in fracture repair). We will define their gene expression and will study how PTH changes their fates and activities in ways that may clarify the anabolic action of PTH and its limitations. We will also study bone lining cells to identify how PTH activates these cells to become active osteoblasts. In Aim 2, we will study the roles of class IIa histone deacetylases (HDACs) in mediating PTH's action to suppress expression of sclerostin, a powerful inhibitor of wnt signaling secreted by osteocytes. We have shown that these HDACs mediate PTHrP's action in chondrocytes and hypothesize that, as in chondrocytes, HDACs respond to PTH by moving to the nucleus and suppressing transcription by Mef2 transcription factors. We will test this hypothesis using an osteocyte cell line from Dr. Divieti Pajevic and using knockout mice. We view sclerostin regulation as a model for many actions of PTH on all cells of the osteoblast lineage. Thus, by defining early cells of the lineage and a novel mediator of PTH signaling, we hope to clarify the mechanisms that can lead to more effective therapies.